In the game, what actually happened was, after I castled (18. ... O-O) and his immediate move (19. Bd4), my rook disappeared and my king moved (by itself) to h8 instead of g8! This also explains the "glancealot CHECKMATED" at the end, when i am NOT checkmated according to the PGN file when you replay the game, see picture below

This would repeatedly happen when i play against this guy, my pieces always go missing after I castle. It has not happened against any other player.

here are two more examples against this guy, i resigned in both games because my pieces went missing!

While disappearing rooks in 960 on this site is a known glitch, so far as I know it is infrequent (happens maybe every 30 games or so).

Yours happening so frequently is worthy of your posting in the bugs/glitches topic. I doubt very, very much if your opponent has found a way to hack chesscube and increase the frequency of this glitch, could just be an unusual happening (once every thousand years?).